Vertical bipolar transistor with recessed epitaxially grown intrinsic base region

ABSTRACT

A method of manufacturing a vertical bipolar transistor including the steps of: providing a semiconductor substrate including a first region of a first conductivity type; forming an extrinsic base region of a second conductivity type in the surface of the first region, the extrinsic base region generally bounding a portion of the first region; forming by ion implantation a linking region of the second conductivity type in the surface of the bounded portion of the first region so as to electrically link generally opposing edges of the extrinsic base region through the linking region; forming an insulating spacer over the junction between the extrinsic base region and the linking region so as to generally bound a portion of the linking region within the portion of the first region; etching the surface of the bounded portion of the linking region a short distance into the linking region; forming by epitaxial growth a first layer of semiconductor material of the second conductivity type on the etched surface of the bounded portion of the linking region; heating the semiconductor substrate to form an intrinsic base region at least partially within the first layer and to form an electrical connection between the intrinsic and extrinsic base regions through the linking region; and forming a second region of the first conductivity type in the surface of the intrinsic base region.

RELATED APPLICATIONS

The present invention is related to copending U.S. patent applicationSer. No. 405,508, filed Sep. 11, 1989, now U.S. Pat. No. 5,003,207 byRosenberg et al., and to U.S. Pat. Ser. No. 309,510, filed Feb. 13, 1989by Harame et al. now U.S. Pat. No. 5,024,957.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor devices andmore specifically to a vertical bipolar transistor including a recessed,epitaxially grown intrinsic base region.

BACKGROUND OF THE INVENTION

In the fabrication of vertical, high performance bipolar transistors, itis generally desirable to provide very shallow intrinsic base regions.Such shallow intrinsic base regions, preferably in the sub-microndimension range, permit the fabrication of extremely high-frequencytransistors: i.e., having speeds in the gigahertz switching range.

The formation of the intrinsic base region in the transistors describedabove is typically accomplished by diffusion from a gaseous or soliddiffusion source, or by ion implantation (I/I). Diffusion tends to bedifficult to control, and may result in undesirably thick base regions.Ion implantation, while more controllable than diffusion, also hasinherent limitations. Particularly with respect to ion implantation ofboron (B) atoms, the thickness of the implanted region is limited bysecondary channeling effects. Further, both diffusion and ionimplantation are particularly difficult to utilize with transistorshaving shallow, heterojunction base regions: e.g. silicon-germanium(SiGe) base regions.

A further challenge faced in manufacturing transistors with narrowintrinsic base regions is that of providing a reliable electricalconnection to the intrinsic base region. Such connections are typicallyaccomplished through the use of an extrinsic base region--a thick,highly doped region disposed in contact with the edge of the intrinsic(or thin, active) base region. As the intrinsic base region decreases inthickness, the linkup between extrinsic and intrinsic base regionsbecomes more difficult to reliably establish. One typical faultencountered in making such a linkup is that of the extrinsic base andemitter regions butting so as to cause an unacceptably low base-emitterbreakdown voltage.

With respect to the formation of thin, intrinsic base regions, a lowtemperature, ultra-high vacuum, chemical vapor deposition (UHV/CVD)process is known in the art for forming thin, discretely doped layers ofepitaxial silicon. See Meyerson, B., "Low Temperature Silicon Epitaxy byUltrahigh Vacuum/Chemical Vapor Deposition," Appl. Phys. Lett. 48(12),24 Mar. 1986, pgs. 797-799. This process has also been used to form SiGelayers, as well as various device regions, including, in certain limitedconfigurations, the base regions of transistors. The process isadvantageous for these purposes in that it provides relativelydefect-free, thin layers. Intrinsic base regions formed using thisprocess, however, may be particularly difficult to connect with theextrinsic base.

Further known in the art is the use of a linker region for electricallylinking an intrinsic base region to an extrinsic base region. A linkerregion, typically a region more lightly doped than the extrinsic baseregion, is formed proximate the junction between the extrinsic andintrinsic base regions. The purpose of the linker region is to insure agood electrical connection between the two regions. Linker regions areparticularly useful where the spacing between the intrinsic andextrinsic base regions may vary, as, for example, where a sidewallspacer is used to set such spacing.

With respect to linker regions, Sugiyama, M., et al , "A 40GHz f_(T)Bipolar Transistor LSI Technology," IEDM 89 221-224, shows a linkerregion out-diffused from a borosilicate glass (BSG) sidewall spacer, thespacer subsequently functioning as an insulator. The use of BSG as adiffusion source is particularly difficult to control, and thus thisprocess is unlikely to yield high performance transistors.

Japanese Kokai No. 62-293,674 (1987) by Matsushita Electric IndustrialCo., Ltd., while not showing linker regions, shows a vertical bipolartransistor wherein an intrinsic base region is formed by implanting ionsinto a recessed surface intermediate bounding, highly doped, extrinsicbase regions. The recessed surface, formed by etching after theextrinsic base regions have been formed, results in the removal of innerportions of the extrinsic base regions. The intrinsic base region issubsequently formed by ion implantation. As discussed above, the use ofion implantation results in an intrinsic base region having a minimumthickness inherent in the process limitations. In the particularstructure shown, the extrinsic base region is in direct contact with theemitter region, which will likely result in substantially degradedperformance. Further, because the etching of silicon is difficult tocontrol, the basewidth and corner doping profile of the transistor willbe difficult to control.

In summary, while high performance, vertical bipolar transistors areknown which provide very fast switching speeds, several technicalproblems need to be addressed in order to continue to advance the art.In particular, the present inventors have recognized the need to provideimproved methods of forming ultra-thin intrinsic base regions, and theneed to provide reliable electrical connections to such thin intrinsicbase regions.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a new and improved highperformance, vertical bipolar transistor, and a method of making thesame, wherein the transistor has a very thin intrinsic base region and aresultantly high switching speed.

Another object of the present invention is to provide a transistor ofthe type described above wherein a reliable electrical connection isprovided between the intrinsic and extrinsic base regions.

Another object of the present invention is to provide such a transistorwherein the base width is not dependent on an etch step and thus ishighly uniform.

Yet another object of the present invention is to provide a transistorof the type described above wherein the doping profile of the intrinsicbase region can be tightly controlled.

A further object of the present invention is to provide a transistor ofthe type described above wherein the base region can be formed using alow temperature process.

Yet another object of the present invention is to provide such atransistor wherein the base region can be formed from heterojunctionmaterials, e.g. SiGe.

Yet a further object of the present invention is to provide a method offabricating a transistor of the type described above, the method beinggenerally compatible with known processing techniques.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a methodmanufacturing a vertical bipolar transistor comprising the steps ofproviding a semiconductor substrate including a first region of a firstconductivity type; etching the surface of the first region to form arecess in the first region; forming by epitaxial growth a layer ofsemiconductor material of a second conductivity type on the etchedsurface of the first region; heating to form an intrinsic base region ofthe second conductivity type at least partially within the layer ofsemiconductor material; and forming a second region of the firstconductivity type in the surface of the intrinsic base region.

In accordance with another aspect of the present invention, there isprovided a vertical bipolar transistor comprising: a semiconductorsubstrate including a first device region of a first conductivity type;a recess in the surface of the first device region; an epitaxial layerdisposed in the recess; an intrinsic base region of a secondconductivity type disposed at least partially within the epitaxiallayer; and a second device region of the first conductivity typedisposed in the surface of the epitaxial layer and contained entirelywithin the intrinsic base region.

DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will become apparent upon a consideration of the followingdetailed description of the invention when read in conjunction with thedrawing Figures, in which:

FIGS. 1-9 are cross-sectional views showing consecutive steps in thefabrication of a vertical bipolar transistor in accordance with thepresent invention; and

FIG. 10 is an enlarged view of a selected portion of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a P- silicon substrate 10 is provided from aconventional crystal melt having, for example, a crystallographicorientation if <100>, and a sheet resistivity in the range of 10-20ohm-cm. It will be understood that "N" and "P" are used herein to denotesemiconductor dopant types, while "+" and "-" are used where appropriateto indicate relative doping concentrations. It will further beunderstood that, while the invention is illustrated with respect tosilicon regions of particular conductivity types, differentsemiconductor materials and other conductivity types can be substitutedtherefor.

Continuing to describe FIG. 1, a layer 12 of N+silicon is formed oversubstrate 10 to a thickness of 2.0 micrometers (μm). A layer 14 ofN-epitaxial silicon is formed over layer 12 to a thickness of 1 μm.Layers 12 and 14 can be fabricated, for example, by first doping theupper surface of substrate 10 heavily N+. A high temperature (i.e.,1100° C.) anneal is performed, and an N- epitaxial silicon layer isgrown over this same surface of substrate 10 using a conventionalepitaxy process. Using such a process, the dopant in the surface ofsubstrate 10 diffuses further downward into the substrate, as well asupward into the epitaxial layer, to provide the structure shown in FIG.1.

Still with reference to FIG. 1, a layer 16 of silicon dioxide (SiO₂,also referred to as an oxide) is formed over layer 14 to a thickness of200 nanometers (nm). Oxide layer 16 can be formed by thermal oxidation.

Referring now to FIG. 2, isolation trench 18 is formed to extend fromthe upper surface of layer 14 downward into substrate 10. Trench 18includes an insulating liner 20, such as an oxide, and a filler material22 such as intrinsic polysilicon. Many processes are known in the artfor forming isolation trench 18, for example: 1) masking and etching toform an open well for the trench, 2) oxidizing the surface of the wellto form oxide liner 20, 3) filling the lined well with polysilicon fill22, 4) planarizing the polysilicon fill, and 5) submitting the structureto a thermal oxidation environment to oxidize the surface of polysiliconfill 22. See U.S. Pat. No. 4,104,086 by Bondur et. al. (assigned to theassignee of the present invention) for another teaching of forming anisolation trench. Trench 18 functions to electrically isolate a deviceregion 24, consisting of registered portions of layers 10, 12, 14bounded by the trench, within substrate 10.

Referring now to FIG. 3, an aperture 26 is formed in oxide layer 16 bymasking and etching the layer, so as to expose a portion of the surfaceof layer 14 over device region 24. A layer 28 of P+polysilicon is formedconformally over the device to a thickness in the range of 300 nm.Polysilicon layer 28 is formed by a conventional CVD process, and can beformed in situ doped with a boron ion species, or can be subsequentlyimplanted with the same type of ions.

Continuing with reference to FIG. 3, a layer 30 of silicon dioxide isformed over layer 28 to a thickness of 80 nm. A layer 32 of siliconnitride (Si₃ N₄, also referred to as a nitride) is formed over layer 30to a thickness of 150 nm. Oxide layer 30 and nitride layer 32 are bothformed by a conventional CVD process.

Still with reference to FIG. 3, a layer 34 of photoresist is formed overthe surface of layer 32, and patterned to provide an aperture 36generally centered over device region 24. Many conventional photoresistmaterials and processes are known in the art, and the exact process useddoes not constitute a part of the present invention.

FIGS. 4-9 show an enlarged portion of the device around aperture 36(FIG. 3) to better illustrate the present invention.

Referring now to FIG. 4, a multistep reactive ion etching (RIE) processhas been used, utilizing patterned resist layer 34 (FIG. 3) as a mask,to form an aperture 38 through layers 32, 30, and 28, consecutively.Nitride layer 32 and oxide layer 30 are etched in CF4 plasma, whilepolysilicon layer 28 is etched using HBr/Cl₂ plasma.

Continuing with reference to FIG. 4, the structure is subjected to aheating or annealing cycle so as to drive dopant from polysilicon layer28 into epitaxial layer 14, whereby to form P+extrinsic base region 40.Subsequent to this thermal treatment, a thin linking region 42 is formedin the surface of the exposed region of epitaxial layer 14. Linkingregion 42 is preferably formed by ion implantation of boron ions, at adosage in the range of 2-4×10¹³ ions/cm², and an energy in the range of4-6 KeV, so as to yield the linker region with a dopant concentration inthe range of 2-5×10¹⁸ atoms/cm³ and a thickness of about 100-200 nm.Linker region 42 is thus lower doped in comparison to extrinsic baseregion 40 (and to a subsequently formed intrinsic base region).

Photoresist 34 is removed in a conventional manner, for example byashing.

Referring now to FIG. 5, thermal oxidation is used to form a layer 44 ofoxide, to a thickness of 50 nm, over the exposed surfaces of polysiliconlayer 28, extrinsic base 40, and linker region 42. Next, a layer 46 ofnitride is formed conformally over the structure by a process of CVD andto a thickness of 100 nm. CVD is further used to form a layer 48 ofoxide conformally over layer 46 to a thickness of 250 nm.

Subsequent to the formation of layers 44, 46, and 48, the device issubjected to an anisotropic RIE etch using a Freon 13 plasma for layer48, a CF₄ /O₂ plasma for layer 46, and a CF₄ plasma for layer 44. Theresult of this etch is the insulating, multilayer (i.e. layers 44, 46,and 48) spacer shown in FIG. 5. In accordance with the presentinvention, the RIE etch described above is continued, using a HBr/Cl₂plasma, for a distance in the range of 50-100 nm into the surface oflinker region 42, forming recess 50 in the surface of linker region 42.

The above-described RIE etch to form recess 50 may result in damage, inthe form of crystal lattice defects, to the surface of linker region 42.Referring now to FIG. 6, in order to cure this damage, the device issubjected to a thermal oxidation environment so as to oxidize the dopedsilicon surface exposed in recess 50, forming thermal oxide layer 52 toa thickness of about 50 nm. This oxidation process is preformed at 700°C. and 10 ATM of pressure.

Referring now to FIG. 7, a wet etch, comprising, for example, BHF, isused to remove oxide layer 52 from the surface of recess 50. As a resultof this wet etch, recess 50 extends in the range of 75-125 nm into thesurface of linker region 42. This wet etch further functions to slightlyetch back oxide layers 44 and 48 relative to nitride layer 46. Thisprocess of forming and removing oxide layer 52 functions to remove anydefects in the surface of linker region 42 resulting from theabove-described RIE process.

Referring now to FIG. 8, a layer 54 is deposited conformally over thestructure so as to form a P type epitaxial silicon region 54A overlinker region 42, and a P type polysilicon region 54B over the remainderof the device. In accordance with the present invention, layer 54 isformed using a low temperature, high vacuum CVD process of the typedescribed in the Meyerson, et al. reference cited herein above. Forexample, this low temperature epitaxial process can comprise subjectingthe device to a gaseous mixture of SiH₄ /H₂ and B₂ H₆ (the dopant), in aflowing gaseous stream, at a temperature in the range of about 500-800degrees centigrade, and a vacuum in the range of about 10⁻⁴ -10⁻² torrduring deposition. Layer 54 is thus formed to a thickness in the rangeof 50-100 nm. As is known in the art, this epitaxial process will form asingle crystal structure 54A over single crystal substrate region 42,and a polycrystalline structure 54B over the remainder of the device.

The distribution of the B₂ H₆ dopant is controlled during this processso as to provide layer 54 with a dopant concentration in the range of3×10¹⁸ -1×10¹⁹, the dopant being more heavily concentrated towards thevertical center of the layer.

In accordance with another embodiment of the present invention,germanium (Ge) ions are introduced into the gaseous stream generated toform layer 54, for example through the use of GeH₄, so as to form a SiGeheterojunction at the interface between linker region 42 and epitaxialregion 54A. The present invention, through the user of this lowtemperature process to form intrinsic base region 54A, is particularlywell-suited to form such a heterojunction device.

FIG. 10 represents an enlarged view of the region of FIG. 9 enclosed ina dashed line.

Referring now to FIGS. 9 and 10, a layer 60 of intrinsic polysilicon isformed conformally over the device to a thickness of 200 nm. Polysiliconlayer 60 is formed by a conventional CVD process. Layer 60 is thensubjected to an ion implantation, using, for example, arsenic ions, todope the polysilicon layer to N+.

Subsequent to the formation of polysilicon layer 60, the device issubjected to a heat or annealing cycle, for example at a temperature of850° C. and for a duration of 30 minutes whereby to form the verticalbipolar transistor of FIGS. 9 and 10. As is best visible in FIG. 10,this thermal anneal functions to drive N dopant from polysilicon layer60 into region 54A, forming N type emitter region 62. Substantiallysimultaneously, P type dopant from layer 54A diffuses downward while Ntype dopant from collector regions 12 and 14 diffuse upward to overcomelinker region 42 beneath intrinsic base region 64. Emitter region 62 isthus formed in epitaxial layer 54A. Intrinsic base region 64, dependingon the exact pre-anneal dopant concentration and profile and the exactthickness of epitaxial layer 54A, may be formed entirely withinepitaxial region 54A, or may extend a short distance into layer 14 (inthe manner shown). Intrinsic base region 64 has an ultra-thin, uniformwidth in the range of 30-80 nm. Polysilicon region 54B is overcome by Ntype dopant from layer 60, becoming part of the polysilicon emittercontact.

During the last-described thermal anneal, linker region 42 forms areliable electrical connection linking extrinsic base region 40 withintrinsic base region 64. In contrast to the background referencesdiscussed above, emitter region 62 is contained entirely withinintrinsic base region 64, with the base region having sufficient cornerdoping to inhibit punch-through defects between the emitter region andcollector region 14.

Layers 60 and 54 are subsequently patterned, using a conventional RIEprocess, to define the regions shown in FIG. 9. In a conventionalmanner, metal contacts (not shown) are provided to polysilicon emitterregion contact 60, polysilicon extrinsic base contact 28, and tocollector region 12. A conventional, highly-doped reachthrough region(not shown) can be used to complete the electrical connection to buriedcollector region 12.

There is thus provided a vertical bipolar transistor, and a method ofmaking the same. The transistor has an ultra-thin, highly uniformintrinsic base region, permitting switching speeds estimated to be inthe 40-80 GHz range. Further, a reliable electrical link is providedbetween the intrinsic and extrinsic base regions. The emitter region isfully contained within the intrinsic base region, inhibitingemitter-collector punch-through. The method of manufacturing thetransistor utilizes low-temperature processes to form the critical,ultra-thin intrinsic base region, providing a well-controlled junctionprofile. This low-temperature process is also particularly useful forforming a SiGe heterojunction device. The present invention hasapplication with respect to the formation of vertical bipolartransistors, and particularly with respect to the formation of suchtransistors for very large scale integrated circuit (VLSIC) devices.

While the invention has been shown and described with respect tospecific embodiments, it is not thus limited. Numerous modifications,changes and improvements within the scope of the invention will occur tothose skilled in the art.

What is claimed is:
 1. A method of manufacturing a vertical bipolartransistor comprising the steps of:providing a semiconductor substrateincluding a first region of a first conductivity type; etching thesurface of said first region to form a recess in said first region;forming by epitaxial growth a layer of semiconductor materials of asecond conductivity type on said etched surface of said first regionsaid layer comprising a layer of single crystal structure on at leastsaid recess, and heating to form an intrinsic region of said firstconductivity type in the surface of said intrinsic base region whilesubstantially simultaneously forming a second region of said firstconductivity type in the surface of said intrinsic base region.
 2. Amethod in accordance with claim 1 wherein said step of forming byepitaxial growth said layer of semiconductor material comprises formingsaid layer of semiconductor material by chemical vapor deposition at atemperature less than about 800 degrees centigrade.
 3. A method inaccordance with claim 2 wherein said layer of semiconductor material hasa thickness in the range of 50-100 nm.
 4. A method in accordance withclaim 1 and further including the steps of:forming an extrinsic baseregion of said second conductivity type in the surface of said firstregion, said extrinsic base region generally bounding a portion of saidfirst region; forming by ion implantation a linking region of saidsecond conductivity type in the surface of said bounded portion of saidfirst region; said intrinsic base region formed within the boundedportion of said first region and linked to said extrinsic base region bysaid linking region.
 5. The method of claim 4 wherein said step offorming said extrinsic base region comprises the steps of:forming anextrinsic base contact of a conductive material containing dopant ofsaid second conductivity type over said first region; and heating saidsubstrate to drive dopant from said extrinsic base contact into saidfirst region.
 6. The method of claim 5 wherein said step of forming aninsulating spacer comprises the steps of:forming a layer of insulatingmaterial generally conformally over said extrinsic base contact and saidbounded portion of said first region; and reactive ion etching saidlayer of insulating material so as to form said insulating spacer. 7.The method of claim 6 wherein said step of etching the surface of saidportion of said linking region comprises the steps of:continuing saidreactive ion etching step into the surface of said bounded portion ofsaid linking region; oxidizing the surface of said bounded portion ofsaid linking region to form a layer of oxide; and removing said layer ofoxide; such that any defects in the surface of said bounded portion ofsaid linking region caused by said reactive ion etching step aresubstantially removed by said oxidizing and removing steps.
 8. Themethod of claim 2 wherein said intrinsic base region comprises asilicon-germanium compound.
 9. The method of claim 1 wherein said stepof forming said second region comprises forming a layer of materialcontaining dopant of said first conductivity type over the surface ofsaid layer of semiconductor material, said heating step driving dopantfrom said layer of material into said layer of semiconductor material toform said second region.
 10. A method of manufacturing a verticalbipolar transistor comprising the steps of:providing a semiconductorsubstrate including a first region of a first conductivity type; formingan extrinsic base region of a second conductivity type in the surface ofsaid first region, said extrinsic base region generally bounding aportion of said first region; forming by ion implantation a linkingregion of said second conductivity type in the surface of said boundedportion of said first region so as to electrically link generallyopposing edges of said extrinsic base region through said linkingregion; forming an insulating spacer over the junction between saidextrinsic base region and said linking region so as to generally bound aportion of said linking region within said portion of said first region;etching the surface of said bounded portion of said linking region ashort distance into said linking region; forming by epitaxial growth afirst layer of semiconductor material of said second conductivity typeon said etched surface of said bounded portion of said linking region,said first layer comprising a layer of single crystal structure on atleast said recess, and heating said semiconductor substrate to form anintrinsic base region at least partially within said first layer and toform an electrical connection between said intrinsic and extrinsic baseregions through said linking region while substantially simultaneouslyforming a second region of said first conductivity type in the surfaceof said intrinsic base region.
 11. The method of claim 10 wherein saidstep of forming said extrinsic base region comprises the stepsof:forming an extrinsic base contact of a conductive material containingdopant of said second conductivity type over said first region; andheating said substrate to drive dopant from said extrinsic base regioninto said first region.
 12. The method of claim 11 wherein said step offorming an insulating spacer comprises the steps of:forming a layer ofinsulating material generally conformally over said extrinsic basecontact and said bounded portion of said first region; and reactive ionetching said layer of insulating material so as to form said insulatingspacer.
 13. The method of claim 10 wherein said step of forming byepitaxial growth a first layer is performed at a temperature of lessthan about 800 degrees centigrade.
 14. The method of claim 13 whereinsaid first layer is formed to have a thickness of the range of 50-100nm.
 15. The method of claim 13 wherein said intrinsic base regioncomprises a silicon-germanium compound.
 16. The method of claim 10wherein said step of forming said second region comprises forming alayer of material containing dopant of said first conductivity type overthe surface of said layer of semiconductor material, said heating stepdriving dopant from said layer of material into said layer ofsemiconductor material to form said second region.
 17. A method ofmanufacturing a vertical bipolar transistor comprising the stepsof:providing a semiconductor substrate including a first region of afirst conductivity type; forming an extrinsic base region of a secondconductivity type in the surface of said first region, said extrinsicbase region generally bounding a portion of said first region; formingby ion implantation a linking region of said second conductivity type inthe surface of said bounded portion of said first region, forming aninsulating spacer over the junction between said extrinsic base regionand said linking region so as to generally bound a portion of saidlinking region with said bounded portion of said first region, etchingthe surface of said linking region to form a recess in said linkingregion; forming by epitaxial growth a layer of semiconductor material ofsaid second conductivity type on said etched surface of said linkingregion, said steps of etching the surface of said first region andforming by epitaxial growth said layer of semiconductor material beingperformed in said bounded portion of said linking region; heating saidsemiconductor substrate to form an intrinsic base region of said secondconductivity type at least partially within said layer of semiconductormaterial, said intrinsic base region formed within the bounded portionof said first region and linked to said extrinsic base region by saidlinking region, and forming a second region of said first conductivitytype in the surface of said intrinsic base region.
 18. A method ofmanufacturing a vertical bipolar transistor comprising the stepsof:providing a semiconductor substrate including a first region of afirst conductivity type; forming an extrinsic base region of a secondconductivity type in the surface of said first region, said extrinsicbase region generally bounding a portion of said first region, whereinsaid step of forming said extrinsic base region comprises the steps of:a) forming an extrinsic base contact of a conductive material containingdopant of said second conductivity type over said first region; and b)heating said substrate to drive said dopant from said extrinsic baseregion into said first region; forming by ion implantation a linkingregion of said second conductivity type in the surface of said boundedportion of said first region so as to electrically link generallyopposing edges of said extrinsic base region through said linkingregion; forming an insulating spacer over the junction between saidextrinsic base region and said linking region so as to generally bound aportion of said linking region within said portion of said first region,wherein said step of forming an insulating spacer comprises the stepsof: a) forming a layer of insulating material generally conformally oversaid extrinsic base contact and said bounded portion of said firstregion; and b) reactive ion etching said layer of insulating material soas to form said insulating spacer; etching the surface of said boundedportion of said linking region a short distance into said linkingregion, wherein said step of etching the surface of said bounded portionof said linking region comprises the steps of: a) continuing saidreactive ion etching step into the surface of said bounded portion ofsaid linking region; b) oxidizing the surface of said bounded portion ofsaid linking region to form a thin layer of oxide; and c) removing saidthin layer of oxide, such that any defects in the surface of saidbounded portion of said linking region caused by said reactive ionetching step are substantially removed by said oxidizing and removingsteps; removing by epitaxial growth a first layer of semiconductormaterial of said second conductivity type on said etched surface of saidbounded portion of said linking region; heating said semiconductorsubstrate to form an intrinsic base region at least partially withinsaid first layer and to form an electrical connection between saidintrinsic and extrinsic base regions through said linking regions; andforming a second region of said first conductivity type in the surfaceof said intrinsic base region.